MySQL UNION: Definition, Syntax, and Examples

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MySQL UNION

The UNION operator in MySQL is used to combine results from multiple SELECT queries into a single result set.

It removes duplicate rows by default.
The number of columns and their data types must be the same in all SELECT statements.

UNION Syntax

SELECT column1, column2 FROM table1
UNION
SELECT column1, column2 FROM table2;

UNION ALL Syntax

If you want to keep duplicate records, use UNION ALL:

SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2;

Note

The column names in the result-set are usually equal to the column names in the first SELECT statement.

SQL UNION Example

We have two tables:

Students_BranchA Table:

student_id	name	course	score
1	Alice	Math	85
2	Bob	Science	70

Students_BranchB Table:

student_id	name	course	score
3	Charlie	Math	90
4	David	English	60

The following SQL statement Combining Students from Two Branches(without duplicates)

Example

SELECT student_id, name, course FROM students_brancha

UNION

SELECT student_id, name, course FROM students_branchb;

Try it yourself

Output:

student_id	name	course
1	Alice	Math
2	Bob	Science
3	Charlie	Math
4	David	English

SQL UNION ALL Example

The following SQL statement Combining Students from Two Branches(with duplicates)

Example

SELECT course FROM students_brancha

UNION ALL

SELECT course FROM students_branchb;

Try it yourself

Output:

course
Math
Science
Math
English

Math appears twice because UNION ALLdoes not remove duplicates.

SQL UNION With WHERE

The following SQL statement combine students with score above 80 from both tables.

Example

SELECT student_id, name, course, score

FROM students_brancha

WHERE score > 80

UNION

SELECT student_id, name, course, score

FROM students_branchb

WHERE score > 80;

Try it yourself

Output:

student_id	name	course	score
1	Alice	Math	85
1	Charlie	Math	90

Explanation:

Alice from BranchA and Charlie from BranchB have scores above 80.
The result is combined using UNION, removing duplicates.

When to Use UNION?

Merging Data → Combine results from multiple tables.
Avoiding Duplicates → UNION removes duplicates automatically.
Keeping Duplicates → Use UNION ALL if duplicates should be retained.
Combining Different Conditions → Use it to fetch data with different WHERE conditions.

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